This invention relates to a spread spectrum communications system, more particularly to a system comprising a transmitter and a receiver linked by a communication channel.
Applications of communications systems employing the spread spectrum (SS) technique are growing in a wide range of fields, including mobile communications and satellite communications, because of the security of SS systems, their resistance to jamming, their compatibility with other existing communications systems, and other features.
FIG. 2 shows an example of a conventional baseband SS communications system. The communications system in this figure comprises a transmitter 12 and a receiver 13 linked by a communication channel 6. The transmitter 12 comprises a data modulator 2 for modulating transmit data received from an input terminal 1, an SS code modulator 4 for modulating a spread spectrum (SS) code received from an input terminal 3, and a multiplier 5 for multiplying the output from the data modulator 2 and the output from the SS code modulator 4 together. The receiver 13 comprises an SS code modulator 8 for modulating an SS code received from an input terminal 7, a multiplier 9 for multiplying the signal received from the communication channel 6 and the output from the SS code modulator 8 together, an integrate-and-discharge filter 10, and an output terminal 11.
The operation of this SS communications system will be described next. The explanation begins with the operation of the transmitter 12.
A series of transmit data {b.sub.m } (where m=-.infin., . . . , -1, 0, 1, . . . , +.infin.) is input in sequence from the input terminal 1 and converted by the data modulator 2 to the transmit data signal: ##EQU1## wherein b.sub.m satisfies the relation b.sub.m .epsilon.{-A, A}, A being a positive real number, and g(t) is the data pulse waveform: ##EQU2## where T is the data pulse duration. At the same time, a spread spectrum code (SS code) {a.sub.l } with period N.sub.c is input from the input terminal 3 and converted by the SS code modulator 4 to the SS signal a(t): ##EQU3## where a.sub.l .epsilon. {-1, 1} and g.sub.c (t) is the SS code pulse waveform: ##EQU4## T.sub.c is the duration of the SS code pulse and satisfies the relation N.sub.c =T/T.sub.c. The transmit data signal b(t) and the SS signal a(t) are multiplied together in the multiplier 5 and the resulting output d(t) is sent on the communication channel 6: ##EQU5##
Next, the receiver 13 performs the following process. First it receives the transmitted signal r(t) via the communication channel 6. It also inputs an SS code {a.sub.l } (the same code as used in the transmitter) with period N.sub.c from the input terminal 7. This SS code is converted by the SS code modulator 8 to exactly the same SS signal a(t) as used in the transmitter, described by Eq. (3). The multiplier 9 multiplies the signals r(t) and a(t) together, and the resulting output r(t)a(t) is input to the integrate-and-discharge filter 10, the output Z from which is used to restore the original transmit data series {b.sub.m }.
The spread spectrum communications system described above suffers from the following problems.
The power spectrum density of the signal d(t) transmitted to the communication channel 6, given by Eq. (5), has comparatively large peak values, as shown in the Denshi Tsushin Gakkai Rombunshi (B), Vol. J66-B, 11 (November 1983), pp. 1362-1369. It therefore interferes strongly with other existing communications systems. The above system also provides inadequate security, despite the listing of security as a feature of SS communications systems. Using pulse detector receiving equipment, for example, it is comparatively easy to intercept the transmission even without knowing the SS code {a.sub.l }.